Discovery of Transposable Elements

Hi in this video we're gonna be talking about the discovery of transposing elements. So when we talk about discovery of transposing elements, the important name to know is Barbara mcclintock and she was the one who first discovered them in the forties and fifties when she was studying this. So first what is a transpose herbal element? Transportable elements are pretty much just small D. N. A segments that can jump or move throughout the genome. So they exist in the genome and they're just sitting there chilling out and then they're like oh I don't want to be in this chromosome anymore, I want to go over there so they just cut it themselves out and move over there. And so transposing elements are found in nearly every organism including you including humans. They exist in us as well. And so how they were discovered is mcclintock studied chromosomal breakage and she did so in maize which in case you're unfamiliar with what mazes it's corn. And so what she found is when she was studying this chromosome breakage, she found that chromosome nine, which is just one of the chromosomes in corn, it tended to break in the same exact spot a lot. And that was very weird because chromosome breakage is an event that happens but it's kind of random right? It's supposed to be random. You know, something weird causes the chromosome to break. But in this case this chromosome just broke all the time in the exact same spot. And she was like, well this is very odd. So she's trying to figure out what factors caused this breakage. And she found two of them the first is called disassociation factor or D. S. And this is a factor located where the break occurred. Often the second factor she found was the activated factor and the activator factor was unlinked so it wasn't nearby Ds. But it was still present in the genome and it was responsible for controlling breakage. So if A. C. Wasn't there the breakage never happened. And so she also found that the A. C. Element was impossible to map. So no matter how many times she tried to you know do all these recombination and calculate the recombination frequency and do everything that we've talked about before in our mapping chapters. She couldn't do it every time she did it it was in a new location. So she thought the A. C. Was potentially moving around. Like this is very odd. And so what she found was that the D. S. And the A. C. Um we're dependent on the other. So D. D. S. Is a non autonomous element. And this means that this element cannot move or cause this break without assistance. So it's there. But a. C. Isn't it's not gonna do anything. It needs a C. To be able to cause this breakage or jump around the genome. The autonomous element is a. C. And this can move. It can act it doesn't need anything else, it completely acts on its own accord. So if we have a C. D. S. Can move. But if we don't have a C. D. S. Cannot meaning that D. S only moves if A. C. Is present. So this is non autonomous while A. C. Is autonomous. Now She was studying corn right? And and it wasn't exactly easy in the 1940s just to be able to sequence the genome right? It wasn't actually feasible. So you had to be able to have some kind of phenotype to realize what was going on. And so she noticed that these corn kernels had an unstable phenotype that would change after a development. So what this looks like is this so you have this white cornel and this was sort of a normal phenotype. The other normal phenotype which isn't on here was just full purple. So these were the normal phenotype. But what she noticed is that she ended up getting a bunch of abnormal phenotype that were very different in what she called unstable. And they're kind of spotted right there. You can see that they may be started out white but then at some point they switched they were unstable. They switched to purple and then you get these like purple tiny purple spots. You can get the majority of it being purple and it sort of creates these swirls and it's different for every corn kernel and that was very odd and she wasn't sure what was going on with the genotype because she hypothesized that the purple had this phenotype or this genotype, right? Where it was dominant, it could have been this or this. And she suspected that the white was essentially recessive, right? Where you had to have this. But she was like what in the world is going on with these white and purple spots? Like what is this genotype that's causing this unstable phenotype and how is it being regulated so that it's so different in every single kernel. And so she did a lot of experiments to prove this. But essentially what she came up with is that there are the three elements you have your A C. You have your D. S. And you have your gene and this encodes for the color. And when everything is just sort of sitting in its original locations, you get a purple colonel, right? Because this gene is undisturbed. But what happens if you have a C. That's going to stimulate Diaz to jump and when DS jumps into the sea gene that's going to inactivate it. And when this color gene is inactivated, it's white, right? So the DS is here and obviously this isn't gonna work because it's now sitting straight in the middle of this gene, so you get a white colonel. But what happens with these spotted? Well what happens is actually they start out white. So they start out like this. So this is kind of the first step. So when the kernel starts dividing it has this Ds element smack dab in the middle of this color jean. But because A. C. Is still present, what happens is that D. S. Can jump again And when it does it reverts back to the normal purple phenotype. And so you start out with a white curdle and then the D. S. Element jumps. And then those regions can can become purple now. And so what the timing of that jump and at what point in development it is can depend on what this color is right? Because the color is going to depend on the exact timing of when that DS element jumps and it's going to be very different for each kernel. And so that is how you get this unstable phenotype of these different colors coming from this really just one gene that could either be purple or white. And so she did a lot of controls with this too, right? Because if A. C. Isn't present right? If A. C. Isn't present here, the Ds. Is gonna stay and that's always gonna be purple. But if the D. The A. C. Is lost here, what's gonna happen is the Ds is always gonna be stuck here and it will always stay white. And so she did a lot of controls of like when the A. C. Was president, what did Ds do when it was absent, what did it do? And she found that you know the Ds element couldn't move without a C. So if the DS. Element was stuck in the gene and A. C. Wasn't there then it's gonna stay there and it's gonna continue to be white. But if the A. C. Elements there it can jump out and revert to purple and cause this unstable phenotype. So that is sort of the discovery of these transposed elements and how they came about. So remember barbara mcclintock and the fact that she studied these unstable phenotype in corn. So with that let's now move on.